Based on an idiom, I have the following as a template to handle timeouts:

{
    local $SIG{__DIE__};
    local $@ = "";
                                                                   
    eval {
        local $SIG{ALRM} = sub { die("alarm\n") };

        alarm(3);
        chomp($input = <STDIN>); # potentially long operation
        alarm(0);

        1;
    } or do {
        die($@) unless $@ eq "alarm\n";
                                                                   
        # timed out
        warn("No answer for three seconds.\n");
        $input = "";
    }
}
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Here, I only want to do_something_more(...) when the alarm is up, but pass on every other $@. So, my if ($@) actually does a little more than your example. Then again, it's subtly different with or do { ... } vs. eval { ... }; if ....

Hope this helps as an example as to why $@ needs to be mucked around :)

Update: Localized $SIG{__DIE__}. Thanks, jplindstrom!</c>